Refractories are materials having properties that make them suitable for use as heat-resistant barriers in high temperature applications. Unshaped refractory materials have the ability to form a joint-less lining, and are often referred to as monolithics. These materials are useful for example as linings for cupola hearths and siphons, in blast furnaces, main, secondary and tilting runners, and more generally in vessels or vessel spouts, ladles, tundishes, reaction chambers and troughs that contain, direct the flow, or are suitable for facilitating industrial treatment of liquid metals and slags, or any other high temperature liquids, solids and gases. Unshaped refractories are typically manufactured in powdered form and mixed with water prior to application. The wet material may be applied as a lining using techniques such as casting, spraying (shotcrete) and gunning followed by setting and drying, prior to firing.
Important properties of refractories are their mechanical, chemical and thermal resistances. Mechanical resistance is commonly assessed using modulus of rupture (MOR; also known as flexural strength) and cold crushing strength (CCS). In the case of refractory materials based on alumina, mechanical improvements can be obtained by the formation of highly mechanically and thermally resistant non-oxide phases (e.g. silicon carbide, silicon nitride, SiAlON), the growth of which is achieved by addition of specific metal containing and non-oxide compounds, and always includes a thermal pre-treatment at controlled temperature in a non-oxidising atmosphere (such as He, N2, Ar, or under vacuum). The skilled person in the art is aware that the presence of specific amounts silicon carbide whiskers in a refractory composition improve the mechanical properties of a finished lining. These whiskers are however difficult to obtain.
U.S. Pat. No. 4,326,040 A discloses refractory compositions formed from (among others) aluminium oxide, silicon carbide and carbon powder wherein the starting materials are fired in the presence of a resin or pitch binder under a non-oxidation atmosphere. The use of resins and controlled atmosphere renders the formation process costly and complex.
U.S. Pat. No. 4,543,345 A discloses refractory compositions formed by hot pressing of, for example, aluminium oxide and silicon carbide whiskers at pressures of at least 28 MPa and temperatures of at least 1600° C. The silicon carbide whiskers are very expensive and extremely carcinogenic and are very dangerous to handle. The dispersion of the silicon carbide whiskers is difficult to achieve and elaborate processing techniques are necessary. With mechanical methods of mixing whiskers and ceramic matrix powders, there is the possibility of whiskers clustering together and whisker damage, and the extent of whisker loading is limited.
U.S. Pat. No. 4,753,764 A discloses formation of a refractory ceramic comprising mixing, for example, silicon carbide powder with whisker formation agents and whisker formation accelerators, moulding the mixture and heat treatment under vacuum and/or Ar at temperatures above 2050° C. WO 91/04234 and EP 0 309 128 A1 also disclose formation of silicon carbide whisker containing refractory compositions involving heat-treatment under inert or non-oxidising atmospheres.
Karamian et al. in J. Eur. Ceram. Soc., 31, 14, 2677-2685, disclose methods for forming refractories comprising as starting materials bauxite, SiC-containing materials, coke and phenol resins. In the firing step, the mixtures were heated under a coke (reducing) atmosphere.
All the above methods of producing various refractory compositions with increased mechanical strength and comprising silicon carbide whiskers require stringent and/or carefully controlled conditions in the formation of the refractories, and especially the heat-treatment step. It is an object of the present invention to provide refractory compositions giving rise to refractory linings having good or improved mechanical properties, and having less complex and expensive formation methods.